The present invention is related to an apparatus and a method of removing a liquid from a rotating substrate. This liquid can be any wet processing liquid as e.g. a wet etching liquid or a cleaning liquid. It can also be a rinsing liquid. The invention is applicable for a number of wet processing steps which are frequently used in the fabrication process of integrated circuits or liquid crystal displays.
The complete and efficient removal of a liquid from a surface of a substrate is a multiply repeated step in e.g. the fabrication process of integrated circuits. Such a step can be performed after a wet etching step or a wet cleaning step or a wet rinsing step or any other step used in said fabrication process wherein said substrate is treated or exposed or immersed in a liquid. Said substrate can be a semiconductor wafer or a part thereof or a glass slice or any other slice of an insulating or conductive material.
The manufacturing of integrated circuits evolves towards processing of each substrate individually rather than in batches of several substrates. In state of the art IC manufacturing, most processing steps as e.g. implantation steps, deposition steps are already performed in a single substrate mode. On the other hand, wet processing steps such as e.g. cleaning steps and subsequent liquid removal steps are typically performed in a batch mode because of lack of appropriate alternatives. Therefore, differences in waiting times are created for each individual substrate between a wet processing step, performed in a batch mode and another processing step, performed in a single substrate mode. Such variability is undesirable with regard to process control. Moreover this mixed batch and single substrate processing increases the cycle time, which is also undesirable. Therefore, there is a general interest in the development of competitive single substrate wet processing steps. Particularly, one of the major challenges regarding single wafer wet processing is a method for removing a liquid from both sides of a substrate. There are two major requirements to be fulfilled for such a method. At first the method should work sufficiently fast. Knowing that in state of the art production lines a substrate is processed typically every 2 to 3 minutes, ideally, in order to avoid equipment duplication, the process step and the liquid removal step should be completed in about such a time frame. Another requirement is related to the preferred substrate orientation. State of the art processing equipment and transportation tools are developed to handle substrates in a horizontal position. Therefore in order to avoid additional substrate handling it would be desirable to perform the wet processing steps using horizontally positioned substrates.
In the European Patent EP 0 385 536 B1, a method is disclosed of drying substrates after treatment in a liquid by pulling said substrate slowly out of said liquid. However, this known method, which is based on the Marangoni principle, requires that the substrates are pulled out of the liquid in an upright position, i.e. a surface of said substrate is about perpendicular to the surface of the liquid bath as can be seen in FIGS. 1 to 6 of the European Patent EP 0 385 536 B1. This handling is incompatible with the majority of the other process steps where the equipment and transportation tools are developed to handle horizontal positioned substrates.
In the U.S. Pat. No. 5,271,774 a spin-drying technique is disclosed which is able to handle horizontal positioned substrates. In fact several small liquid islands are formed being removed from the substrate by a rotary movement. It is known that such a spin-drying technique leaves undesirable residues, often referred to as drying marks, on the substrate surface, particularly on surfaces having mixed hydrophilic and hydrophobic regions.
In the U.S. Pat. No. 5,660,642 a liquid removal technique is disclosed wherein a liquid film present on a surface of a substrate can be removed by applying rinsewater together with a surface tension reducing vapor. Particularly, a disadvantage is that regardless of the precise nature of the liquid, during the liquid removal process always rinsewater is supplied. Furthermore, the surface tension reducing vapor is passively applied, e.g. by natural evaporation, which makes it difficult to locally, i.e. at moving zone, have a good and efficient control on the vapor supply or to direct the vapor. Moreover U.S. Pat. No. 5,660,642 does not disclose how to remove a liquid film substantially simultaneously from two opposite surfaces, i.e. top and bottomside, of a horizontally positioned substrate. Neither does U.S. Pat. No. 5,660,642 disclose how to remove efficiently a liquid from the topside of a horizontally positioned substrate.
In an aspect of the invention a method of removing a liquid from at least one surface of at least one substrate is disclosed, said method comprising the steps of:
supplying a liquid on at least a part of said surface of said substrate;
supplying a gaseous substance to said surface of said substrate, said gaseous substance being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension being lower than that of said liquid; and
subjecting said substrate to a rotary movement. Said gaseous substance can comprise a vaporised substance which is miscible with said liquid and when mixed with said liquid yields a mixture having a surface tension being lower than that of said liquid. Said gaseous substance can comprise a gas which is miscible with said liquid and when mixed with said liquid yields a mixture having a surface tension being lower than that of said liquid. Said gaseous substance can comprise a mixture of a vaporised substance and a gas, like e.g. helium, argon or nitrogen, said mixture being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension being lower than that of said liquid. Particularly, by supplying said liquid and said gaseous substance on said surface of said substrate, at least locally a sharply defined boundary is created between the liquid and the gaseous substance, i.e. a so-called liquid-vapor boundary.
In an embodiment of the invention, said rotary movement is performed at a speed to guide said liquid-vapor boundary over said substrate. Preferably this boundary is a curved boundary. The configuration is such that the liquid is kept at the outerside of the curved boundary, i.e. at the liquid side of the liquid-vapor boundary. In an embodiment of the invention the substrate can spin around its own axis. Alternatively said substrate can also be subjected to a rotary movement where said substrate no longer spins around its own centre.
In another embodiment of the invention, a method of removing a liquid from at least one surface of at least one substrate is disclosed, comprising the steps of:
subjecting said substrate to a rotary movement
supplying a liquid to at least a part of said surface of said substrate; and
supplying a gaseous substance to said surface of said substrate while supplying said liquid, said gaseous substance being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension being lower than that of said liquid. Particularly on at least a part of the surface of the substrate, fresh liquid is sprayed continuously. The entire surface at the liquid side of the liquid-vapor boundary can be covered, as e.g. for hydrophilic surfaces, with a continuous film of the liquid. The speed of the rotary movement is chosen such that the flow of said sprayed liquid on at least one side of the wafer is transported outwards due to the centrifugal forces. Moreover, said gaseous substance, when mixed with said liquid yields a mixture having a surface tension being lower than that of said liquid. The resulting surface tension reduction of said liquid facilitates the movement of said liquid towards an edge of the substrate. The surface left behind is cleaned and dried. It is presumed that this drying action is obtained according to at least the combination of the rotary movement and the Marangoni effect. According to the Marangoni effect said substances will be mixed with the liquid such that in the liquid meniscus, its concentration decreases in the direction of the liquid. This gradient in concentration creates an additional force exerted on the liquid film in the direction of the liquid film resulting in a good drying performance.
The liquid is selected dependent upon the applied wet processing step like e.g. a wet etching step or a cleaning step or a rinsing step. To initiate the drying process, besides said liquid also a gaseous substance reducing the surface tension of said liquid is sprayed on at least one surface of said substrate. Particularly, a pressurized gaseous substance is actively supplied e.g. by using at least one nozzle, said nozzle preferably being movable. Alternatively, instead of a movable nozzle, at least one static inlet can be used for supplying, preferably actively, said gaseous substance on said surface of said substrate. Said surface tension reducing gaseous substance can be isopropyl alcohol (IPA), but also any other gaseous substance which is miscible with said liquid and which will form a mixture with said liquid having a surface tension lower than that of said liquid alone, can be used. Particularly, said gaseous substance may be heated at a temperature typically in the range between 20 and 100 degrees Celsius.
In another embodiment of the invention, eventually prior to the liquid removal step an etching, a cleaning or a rinsing liquid or a sequence of such liquids can be supplied to the entire surface of a rotating substrate. Particularly prior to the liquid removal step of the present invention, a liquid comprising water, ozone and an additive acting as a scavenger can be dispensed on a surface of a rotating substrate in order to remove organic contaminants from said surface. Preferably said liquid is maintained at a temperature below the boiling point of said liquid. The parameters can be optimized such that a liquid film can completely cover a surface. The spinning motion will quickly transport the liquid over the surface towards the edge, thus allowing relatively short carry-over transients and thus also allowing for relatively short rinsing times. Using such a continuously switched flow of liquids eliminates the undesirable passage of liquid-gas interfaces over the surface. The liquid removal method of the present invention is applicable for each sequence of at least one wet processing step by supplying a surface tension reducing gaseous substance together with the liquid. The drying can thus be applied directly on the processing liquid if relevant for the application. Since the proposed drying technique is found to be very fast, process non-uniformity over the surface can be kept very low.
In another embodiment of the invention, eventually prior to the liquid removal process of the present invention, a gas mixture comprising water vapor, ozone and an additive acting as a scavenger can be supplied on a surface of a substrate in order to remove the organic contaminants from said surface. This action is usually followed by a wet processing step, i.e. a rinsing step.
In another embodiment of the invention, another force can be combined with the liquid removal process of the present invention. Particularly, by using megasonic energy as said other force to agitate the liquid applied during the removal process the cleaning performance of said liquid removal process can be enhanced. Doing so can help in particle reduction. Alternatively also contacting a surface with a rotating cleaning pad is an example of such other force.
In an aspect of the invention an apparatus for removing a liquid from at least one surface of at least one substrate is disclosed, said apparatus comprising:
a substrate holder which is subjectable to a rotary movement, said substrate being releasably held by said substrate holder;
a liquid supply system for applying a liquid on at least part of said surface of said substrate;
a gaseous substance supply system for applying a gaseous substance on said surface of said substrate. Preferably, said gaseous substance supply system and said liquid supply system are positioned such that said gaseous substance is applied closer to the centre of said rotary movement of said substrate holder than said liquid. Particularly said liquid supply system is movable relative to said substrate holder.
In an embodiment of the invention, said apparatus further comprises a chamber, preferably pressurizable, wherein said substrate holder is positioned. This chamber is designed in a manner to avoid back splashing of the liquid removed from a surface onto said surface. For instance, a chamber having slanted walls may be used. Particularly, the substrate holder may be positioned horizontally in said chamber. In such case, preferably the vertical walls of said chamber are oriented such that the angle between said walls and the horizontally positioned substrate holder is smaller than 90 degrees in order to prevent back splashing of the liquid which is removed according to the method of the present invention.
In another embodiment of the invention, said apparatus further comprises a generator of megasonic energy and a transmitter for transmitting said megasonic energy to a surface of the substrate via the liquid being supplied at said surface.
In another embodiment of the invention, the gaseous substance supply system can comprise at least one nozzle for applying said gaseous substance on said surface of said substrate and said liquid supply system can comprise at least one nozzle for applying said liquid on said surface of said substrate, said nozzles are positioned such that said gaseous substance is actively applied closer to the centre of the rotary movement of the substrate holder than said liquid. Particularly, at least locally a sharply defined liquid-vapor boundary can be created which is located inbetween a first and a second adjacent nozzle, said first nozzle being part of said gaseous substance supply system, said second nozzle being part of said liquid supply system. Further according to the apparatus of the invention, said nozzles can be mounted on an arm, said nozzles being movable on said arm and/or said arm being movable relative to said substrate holder.
In another embodiment of the invention, the gaseous substance supply system comprises at least one static inlet for applying said gaseous substance on said surface of said substrate and the liquid supply system comprises at least one nozzle for applying said liquid on said surface of said substrate. Particularly, at least locally, a sharply defined liquid-vapor boundary can be created being located between the rotation centre and the liquid supply nozzle being located at the shortest radial distance from the rotation centre. Further according to the apparatus of the invention, said liquid nozzles can be mounted on an arm, said nozzles being movable on said arm and/or said arm being movable relative to said substrate.